"Birth by Sleep" is a video game in the Kingdom Hearts series, released in 2010 for the PlayStation Portable. It serves as a prequel to the main series, exploring the backstory of key characters like Terra, Aqua, and Ventus. Players navigate various Disney-themed worlds, battling enemies and uncovering the game's narrative. The gameplay features a unique combat system that allows players to customize abilities and spells. Each character has their own storyline, which intertwines with the overarching plot of Kingdom Hearts. The game emphasizes themes of friendship, sacrifice, and the struggle between light and darkness.